The basal ganglia, and especially the dopaminergic components of this system, are well known to play a central role in the role in the etiology and pathophysiology of several neurological and psychiatric disorders including Parkinson's disease and schizophrenia. More recently, however, mesotelencephalic dopaminergic systems have also been viewed as integral to certain types of learning and memory, affective responses and perception, and several types of higher cognitive function. In vivo, dopaminergic neurons fire spontaneously at low rates. This activity exists along a continuum of firing pattern from a regular pacemaker-like pattern on one end, to an irregular or random pattern to a slow bursty pattern on the other end. Dopaminergic neurons in vivo typically respond to behaviorally relevant environmental stimuli with an increase in firing rate in the form of a low frequency burst that usually lasts for a few hundred milliseconds. The timing of the dopaminergic signal is crucial for many of the functions ascribed to the dopaminergic system in signaling stimulus characteristics, reward salience or predictive error. Although it is clear that switches to the different patterns of activity are triggered by afferent activity, the afferents responsible and in particular the mechanisms of the burst or burst initiation are not clear. It is the overall goal of this competing renewal to extend observations made in the last Brant cycle by concentrating on GABAergic mechanisms in the afferent control of substantia nigra dopaminergic neurons studied by in vivo and in vitro neurophysiology, light and electron microscopy and in vivo microdialysis. There are 5 specific aims that will test the following hypotheses: (1) GABA-A receptors on dopaminergic neurons re predominantly or exclusively activated by GABAergic inputs in vivo under typical experimental conditions and activation of GABA-B receptors only occurs when the GABA transporter is saturated by excessive or high frequency Input and/or pharmacological blockade, (2) Most postsynaptic GABA-B receptors on substantia nigra dopaminergic neurons are located perisynaptically, (3) Afferent induced alterations in the pattern of activity of DAergic neurons lead to significant changes in extracellular levels of DA in striatum and substantia nigra, (4) Nigral GABAergic interneurons ore a source of afferent input to DAergic neurons, and (5) The difference in sensitivity to GABA-A receptor agonists between DAergic and GABAergic neurons in substantia nigra is due to a differential GABA-A subunit composition and/or a difference in the density of GABA-A receptors. These data should provide answers to several important questions about the afferent control of nigral dopaminergic neurons which are essential for understanding the normal function of the basal ganglia and which may also point the way toward improved pharmacotherapies for disorders involving the dopamine system.